1. Field of the Invention
The present invention relates to a power supply noise reduction circuit and a power supply noise reduction method, and more particularly to a power supply noise reduction circuit and a power supply noise reduction method that reduce power supply noise generated by switching a current source.
2. Description of the Related Art
Today, as LSI (Large scale integration circuit) chips get faster, power supply noise reduces the margin of the design of delay, sometimes causing an analog circuit to malfunction.
More specifically, a current flow, which is generated each time a transistor such as a CMOS (Complementary metal oxide semiconductor) is switched on or off, causes a fluctuation in the power supply. Such a power supply fluctuation, if any, could change the delay amount of a gate in a logical path and, as a result, the delay of the path itself changes depending upon the amount of power supply fluctuation. When a circuit is designed, this fluctuation amount must be taken into consideration. However, when the power supply fluctuation amount cannot be suppressed to a desired value, the delay margin is decreased and, in the worst case, there is no window available for use in transmission.
In an analog circuit, the power supply is sometimes used to create a medium potential that is used in the circuit. Therefore, if this medium potential changes, the behavior of the circuit itself changes with a possibility that the circuit does not operate as designed.
An increase in the speed of an LSI and a decrease in the power supply voltage tend to increase a change in the current amount. As the change in the current amount increases, the noise amount (power supply fluctuation amount) ΔV (proportional to the product of inductance component L of power supply and current change di/dt) also increases.
Conventionally, an on-chip decoupling capacitor is mainly used to reduce such power supply noise. The capacitance of a gate is usually used as the decoupling capacitor. However, to allow this decoupling capacitor with the capacitance of a gate to process noise including a high-frequency noise, there is a tradeoff between the necessity to reduce the gate length and the decrease in the capacitance efficiency per unit area.
On the other hand, an increase in the on-chip capacitors results in an increase in the chip size and increases the chip cost. In view of this, there is a need for a method, other than the method of increasing on-chip capacitors, for efficiently suppressing the power supply fluctuation at a low cost.
One of the inventions for reducing this noise is disclosed in Patent Document 1. The invention disclosed in Patent Document 1 relates to a bypass capacitor on a printed circuit board or an LSI package. In this Patent Document 1, because the effect of a capacitor is weakened if a power supply bypass capacitor is away from an LSI, a series resonance circuit is inserted in parallel to the bypass capacitor to decrease the impedance for the noise of a specific frequency. This configuration allows the noise component corresponding to the setup “specific frequency” to pass through the series resonance circuit, places the bypass capacitor virtually nearer, and suppresses the fluctuation in the power supply potential.
Other related art technologies for this problem are disclosed in Patent Documents 2-4.
[Patent Document 1] Japanese Patent Laid-Open Application No. 2002-83920 (paragraphs 0007-008, FIG. 1)
[Patent Document 2] Japanese Patent Laid-Open Application No. 2003-258612
[Patent Document 3] Japanese Patent Laid-Open Application No. Hei 06-138241
[Patent Document 4] Japanese Patent Laid-Open Application No. Hei 07-321470
However, according to the invention disclosed in Patent Document 1, it is difficult to design filtering for power supply noise unless the noise components are known, which is usually difficult to predict. Actually, however, because a noise includes many frequency components and what frequency noise affects an LSI depends on the LSI operation, it is difficult to predict it in the design stage. Therefore, an exemplary problem is that the noise cannot be decreased sufficiently. Patent Documents 2-4 also fails to disclose means for solving this problem.